Coating finishes, particularly exterior coating finishes in the automotive industry, are generally applied in two or more distinct layers. One or more layers of primer coating composition may be applied to the unpainted substrate first, followed by one or more topcoat layers. Each of the layers supplies important properties toward the durability and appearance of the composite coating finish. The primer coating layers may serve a number of purposes. First, the primer coating may be applied in order to promote adhesion between the substrate and the coating. Secondly, the primer coating may be applied in order to improve physical properties of the coating system, such as corrosion resistance or impact strength, especially for improving resistance to gravel chipping. Third, the primer coating may be applied in order to improve the appearance of the coating by providing a smooth layer upon which the topcoat layers may be applied. The topcoat layer or layers contribute other properties, such as color, appearance, and light stabilization.
In the process of finishing the exterior of automobiles today, metal substrates are usually first coated with an electrocoat primer. While the electrocoat primer provides excellent surface adhesion and corrosion protection, it is often desirable to apply a second primer layer. The second primer layer provides additional properties not available from the electrocoat primer. Resistance to gravel chipping is one of the critical properties provided by the second primer layer. The second primer layer may also enhance the corrosion protection of the finish and provide a smoother surface than the electrocoat primer. The second primer also serves to provide a barrier layer between the electrocoat primer layer, which usually contains aromatic moieties and other materials that can cause yellowing on exposure to sunlight, and the topcoat.
Mitsuji et al, U.S. Pat. Nos. 5,281,655, 5,227,422, and 4,948,829, all of which are incorporated herein by reference, disclose automotive basecoat coating compositions containing polyurethane resin emulsion, a second resin emulsion then can be an acrylic resin, and a crosslinking agent. In Mitsuji '829, the polyurethane resin is prepared by dispersing an isocyanate-functional prepolymer and having the water react with the isocyanate groups to chain-extend the prepolymer. The prepolymer is prepared using an aliphatic diisocyanate, a polyether or polyester diol, a low molecular weight polyol, and a dimethylolalkanoic acid. In Mitsuji '655 and '422, the polyurethane resin is prepared by reacting an aliphatic polyisocyanate, a high molecular weight polyol, a dimethylolalkanoic acid, and, optionally, a chain extender or terminator. Because the Mitsuji patents are directed to basecoat-coatings, these patents provide no direction for preparing compositions that have the chip resistance and other properties required for primer coating layers.
Gessner et al., U.S. Pat. No. 6,437,036, incorporated herein by reference, discloses a thermosetting aqueous primer composition that includes a polyurethane polymer, an acrylic polymer, and a crosslinking component that is reactive with at least one of the polyurethane polymer and the acrylic polymer. The polyurethane polymer has a glass transition temperature of 0° C. or less. The acrylic polymer has a glass transition temperature that is at least about 20° C. higher than the glass transition temperature of polyurethane resin. A composite coating has a cured layer of the primer composition and has at least one topcoat layer. This coating composition has a very low volatile organic content (VOC), but it would be desirable to increase the smoothness of the primer layer. While increased smoothness could be obtained by adding an organic solvent to aid flow, the organic solvent would increase VOC. Further, it would be desirable to remove any solvents that are regulated as so-called hazardous air pollutants (HAPs). Unfortunately, effective cosolvents for stabilizing polymer in water, such as glycol ethers, are being regulated as HAPs.
High solids, solventborne primers have been used for decades in the automotive industry. These primers have been formulated for excellent smoothness, but typically have VOC (volatile organic content) values of 400 to 500 grams per liter. Waterborne primers, such as the Gessner primer, have much lower VOC values of 200 grams per liter or less. The waterborne primers have not provided the smoothness that solventborne primers have provided. It would be desirable, therefore, to have a primer composition that provides improved smoothness, which additionally can be formulated with a very low content of volatile organic solvent.